The known solutions of the above-described type, even if they are widely used, tend to lose their efficiency upon time. In fact, the passage defined by the sleeves is limited to the lower ends thereof, and tends to obstruct due to the presences of boughs, leaves, etc., which inevitably tend to deposit at the bottom of the manifold and enter the sleeves therefrom. When the sleeves lose their draining function, the rainwater overflows from the manifold and tends to infiltrate into the systems of the motor vehicle, e.g. in the aeration system.
Furthermore, it is preferred to avoid the drainage of the rainwater into the engine compartment, since such water, with particularly cold temperatures, tends to freeze and thus damage the electric members present in the engine compartment.